The present invention relates to a technology for improving the efficiency of a power supply unit and, more particularly, to a technology which is effective in reducing the occurrence of a loss due to overcurrent detection in a DC-DC converter.
Examples of an electronic device such as a hard disc drive include one using a DC-DC converter of a PWM (Pulse Width Modulation) type as a stabilized DC power supply or the like.
There is a DC-DC converter of this type which has an overcurrent detecting function for preventing the breakdown of an inductor, the output element of the DC-DC converter, or the like even when a load current enters an abnormal state.
As causes of the abnormal state of the load current, various factors can be considered such as the breakdown of a semiconductor integrated circuit device or a power MOSFET serving as a load, and an overflow of a smoothing capacitor. As the patterns of the abnormal load current also, various states can be assumed such as a load short-circuited state, and an over-load state which is reached with time.
Therefore, unless it can be accurately determined whether the detected load current is a current to be supplied to the semiconductor integrated circuit device in a normal state or a current flowing when any abnormality has occurred, the semiconductor integrated circuit device or the power MOSFET serving as the load may require a breakdown voltage with excess margin for the current ability of the DC-DC converter or, conversely, the element may break down without succeeding in detecting an overcurrent. As a result, a high-accuracy method for detecting an average load current becomes necessary.
As examples of a technology for detecting the average load current, there are known one which detects an average load current, one which detects a power supply current, and the like.
First, according to the technology for detecting the average load current, the average load current is determined by inserting a detection resistor in series between a smoothing inductor and a smoothing capacitor, approximating the voltage between both terminals of the detection resistor to a load current through attenuation of ripple in a fPWM (variable pulse width modulation) cycle therein using a low-pass filter, and making a comparison using a comparator.
Subsequently, the result of the determination by the comparator is outputted to a subsequent-stage digital filter which counts a detection time and, when a set time is reached, a regulator is shut down using a subsequent-stage OC (Over Current) circuit.
On the other hand, in the case of detecting the power supply current, the average load current is equivalently determined by inserting a detection resistor between a power supply and the source of a power MOSFET, attenuating ripple in the fPWM cycle in the voltage between both terminals of the detection resistor using a low-pass filter, and making a comparison using a comparator.
Because a power supply current flows only during the ON time interval of a switching cycle, it has the shape of a rectangular wave, and is approximated to the load current by performing filtering using the low-pass filter mentioned above.